1. Field of the Invention
The present invention relates to a power supply unit for use in the fields of portable appliances such as cellphone and personal computer, power tools such as electric tool and vacuum cleaner, power generation such as for electric car, electric industrial vehicle, electric bike, electric-assisted bicycle, electric wheelchair, and electric robot, system power supplies such as load leveling, peak shift, and backup, and others, and in particular, to improvement in the performance thereof.
2. Description of the Related Art
In contrast to portable appliances such as cellphone and personal computer, appliances in the power tool field such as electric tool and in the power field such as for electric bike demand power supply having high power and high capacity, because fluctuation in the load to power supply is greater and continuous long-term operation is needed.
It is generally difficult to make a battery higher both in capacity and power, because these properties are antinomic to each other. For example, a nonaqueous electrolyte battery such as lithium-ion secondary battery, a typical example of high-capacity secondary battery, allows continuous long-term discharge at a low load of approximately 0.2 C, and thus is mainly used as a power supply unit for portable appliances, but the current density per electrode area at such a low load is only about 0.01 A/cm2. For that reason, such a high-capacity nonaqueous electrolyte battery is not suitable as a high-load power supply that is used in the power field, for example for electric bike, electric-assisted bicycle, and others, that is desirably discharged at large current, specifically at a current density of 0.1 A/cm2 or more. In addition, pulse discharge characteristics at large current is important in the power field, because the power supply is turned on and off frequently, but the high-capacity nonaqueous electrolyte batteries used, for example, in portable appliances do not have favorable pulse discharge capacity. On the other hand, high-power power supplies such as capacitor allow discharge at large current, but the capacity is extremely limited, which makes continuous long-term discharge difficult.
Thus, it has been difficult to obtain a power supply satisfying such requirements in both properties with a single battery, and hybrid power supply units in combination of a long-term small-power power supply and a short-term large-power power supply have been proposed and studied for commercialization. For example, hybrid power supply units using a secondary cell such as lead battery, NiCd battery, nickel metal-hydride battery, lithium ion battery or a fuel cell as the long-term small-power power supply and a capacitor (condenser) as the short-term large-power power supply were proposed (Japanese Unexamined Patent Publication Nos. 8-308103, 2004-48913, and 2004-56995, and WO 2002/025761 Pamphlet).
Japanese Unexamined Patent Publication No. 8-308103 discloses a hybrid power supply unit having a battery, a large-capacity condenser, and a current control circuit, in which the efficiency of the battery is controlled by the current control circuit in such a manner that the battery discharge current supplied to the load at large load becomes as close as possible to the sum of the battery discharge current supplied to the load at low load current and the charge current flowing from the battery to capacitor. Alternatively, Japanese Unexamined Patent Publication No. 2004-48913 proposes a power supply in which an auxiliary power supply is connected to the main battery unit in parallel, a voltage-raising means is connected to the auxiliary power supply in series, and thus, the secondary cell is charged efficiently. Yet alternatively, WO 2002/025761 Pamphlet proposes a hybrid power supply unit having a fuel battery and a unit secondary battery connected in series, as well as a means of detecting residual capacity and a means of controlling a charge for the unit secondary battery. Yet alternatively, Japanese Unexamined Patent Publication No. 2004-56995 proposes a hybrid power supply unit having an electrical double-layer capacitor containing a pair of capacitor terminals, an energy storage device, and first and second bidirectional DC/DC converters.
However, the conventional hybrid power supply units proposed in Japanese Unexamined Patent Publication Nos. 8-308103, 2004-48913, and 2004-56995, and WO 2002/025761 Pamphlet employed the combination of different kinds of power supply units or different kinds of parts, and thus, demanded a complicated control system such as current-controlling circuit, voltage-increasing means, residual capacity-detecting means, charge-controlling means, and bidirectional DC/DC converter, for optimized operation of these different kinds of power supplies and different kinds of parts.